Method and device in an aerial towed hit detector

ABSTRACT

An aerial towed hit detector used together with a sleeve target can obtain an oscillatory movement in the roll angular direction because of the small aerodynamic stability of the target. For determining the path or the miss distance of a projectile passing by the target and the hit detector, the position of the hit detector in the air is measured for allowing a compensation to be made in the determination of hit parameters of the projectile. In the determination of the position of the hit indicator tow accelerometers are used measuring the acceleration laterally or in a tangential direction. One accelerometer is attached to the hit detector and one to the tow rod. By forming the difference of the signals from the accelerometers and integrating the difference signal in two steps information of the angular position of the hit indicator is obtained. This calculation is made in a microprocessor for those times when pressure waves from projectiles hit the hit detector.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a device for determiningthe roll angular position of an aerial towed hit detector, where thedetermination is in particular to be used when towing targets of typenon-rigid targets such as sleeve targets.

DESCRIPTION OF THE RELATED ART

When indicating the positions of hits in test firing at aerial towedtargets, for example of type sleeve targets, conventionally a hitdetector is used, that can be suspended in principle in a cable betweenthe towing aircraft and the target. However, such a hit detector has anot quite stable position, in particular a not stable angular position,but can obtain an oscillating or swinging movement in the roll angulardirection or laterally because of the aerodynamic instability of manytargets, for example targets made of loose flexible sheet material suchas a sleeve target. When making an indication by means of the hitdetector various calculation routines are used, which are performedautomatically and in which generally the position of the hit indicatoris assumed to be known, so that it in many cases is assumed to have forexample the same orientation, that is that some reference line in theindicator is located in a vertical plane. However, because of theoscillatory movement of the hit indicator errors will then be introducedin the calculation. It will become particularly observable in the casewhere hit indicators are made with a high accuracy such as is commonlyused nowadays, for example for hit detectors providing an indicationwithin one of twelve possible angular sectors.

In U.S. Pat. No. 5,247,488 a sensor ring 1 is disclosed comprising atleast three acoustical detectors 2.1-2.8. The ring is assumed to beattached to the body of a towed target, for the case of "tow-target bagsthe sensor ring may be part of the wall of the front side electroniccylinder" (col. 3, lines 25-28). Such a ring has no preferred angularposition or orientation since the tow target can rotate about alongitudinal axis. For ascertaining the rotational position of thesensor ring with respect to the vertical line a vertical sensor 8 isarranged in a fixed connection to the sensor ring 1 (claim 6). Thevertical sensor 8 consists of a mass pendulum 9 and an absolute-anglesensor 10 on a mounting rod 11 extending along a diameter of the sensorring 1 (col. 2, lines 29-31 and FIG. 1). The signal provided by theabsolute-angle sensor 10 is used in calculations of the shot angle.

In the International patent applications having publ. Nos. WO-A179/00452 and WO-A1 91/10876 methods and devices are disclosed havingarrays of acoustical sensors which would greatly benefit frominformation on the angular position of the sensor arrays. However, nosuch orientation sensor means are described in these applications and nohint is given that the provision of such a sensor would favour thecalculations that are performed for indicating various hit parameters.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and a device forimproving the accuracy when indicating various hit parameters, such asmiss distance, by means of hit detectors.

It is another object of the invention to provide a method and a devicefor determining the position of a hit detector that is towed by anaircraft, whereby an increased accuracy can be obtained when indicatinghit parameters in relation to an aerial towed target.

Thus generally, one or more hit parameters are determined for aprojectile passing in the vicinity of a target. The hit parameters canbe miss distance, path or direction of the projectile, etc. The targetis provided with some type of hit indicator or hit detector and thissenses in the conventional way shock waves generated by the projectilepassing through the air. Based on the detected shock waves, calculationsare performed for determining the hit parameters. The position of thehit indicator is determined by position determining means all the timeor at least at those instances when the shock waves are detected. Thisdetermined position is then considered in compensation means in thecalculations of the hit parameters of the projectile. The compensationmeans are preferably incorporated in the means which make thecalculation of the hit parameters. In particular, the angular positionof the hit indicator can be determined in relation to some referencesystem that is geometrically related to the earth, such as in relationto a vertical plane passing in the longitudinal direction of the hitindicator or through the connection between the target and a towingairplane.

In the determination of the position of the angular position of the hitindicator the acceleration of the hit indicator can be measured, forexample in a tangential direction or in a lateral direction thereof, inthe case where it is suspended in the connection between the target andthe towing aircraft, such as that the hit indicator comprises a ratherheavy body, that is is rigidly joined to a tow rod forming a part of thetow connection.

The position determination means can then include one or moreaccelerometers located at suitable places at the hit indicator dependingon the configuration thereof. The hit indicator can as above in theconventional way comprise a rather heavy indicator body that issuspended in a tow connection, by means of which the target is intendedto be towed. The tow connection can comprise a rigid tow rod, which isattached between a tow cable and the target and to which the indicatoris attached. The indicator body will, for such a suspension, performoscillatory movements and an accelerometer can be attached to the bodyof the hit indicator for sensing the acceleration thereof in directionsperpendicular to a plane passing centrally through the hit indicator andthrough the place connecting the indicator body and the tow connectionand in particular through the tow rod. At the place connecting theindicator body and the tow connection then advantageously anotheraccelerometer is attached. It senses the acceleration in the samedirections as the accelerometer attached to the indicator body andprovides information on the acceleration and position of the connectionplace.

Also an indicator body, that is located aligned with the connectionbetween a towing aircraft and the target, for example forms a part ofthe connection of the aircraft and the target, can make oscillatorymovements resulting from the movements of the very target. The angularposition of an indicator body attached in that way can also be measuredby means of suitably located accelerometers.

An alternative method is locating the accelerometers for measuringinstead or in addition centripetal acceleration, that is theacceleration in directions perpendicular to those mentioned above. Sucha location can primarily result from expecting a smooth velocity in thetangential direction, that is a smooth rolling movement, and nosignificant tangential acceleration. A third accelerometer can then berequired for allowing determination of the rotation direction of theindicator body.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which

FIG. 1 is a schematic view from the side of a sleeve target towed by anaircraft and comprising an hit detector,

FIG. 2 is a view from the rear, that is in the flight direction, of atowed sleeve target and a hit indicator comprising measurement devicesfor the angular position,

FIG. 3 is a schematic block diagram of a circuit including a calculationroutine that is required for improving accuracy when indicating hitparameters.

DETAILED DESCRIPTION

In FIG. 1 a sleeve target 1 is shown as seen from the side thereof. Itcomprises in the conventional way a tubular device of a flexible sheetmaterial. The sleeve target 1 is attached, at its front, more narrowopening, to the rear end of a rigid tow rod 1 by means of several wires3. The tow rod 1 is at its front end attached to a tow cable or tow wire7. The tow cable 7 is at its other end is attached to an airplane, notshown, in order to be towed thereby. To the rigid tow rod 1 a hitindicator 9 is attached comprising an elongated, essentially cylindrichit indicator body. The indicator body is attached to the tow rod 1 bymeans of a flat web plate 11, so that the longitudinal direction of thehit detector 9 is parallel to the longitudinal direction of the tow rod5. Usually the tow rod 5, the web plate 11 and the indicator body 9 areall rigid parts that are rigidly attached to each other, so that theassembly of these parts moves as one solid body.

Because of the configuration of the sleeve target 1, primarily becauseit is made of a rather loose material, it will not be aerodynamicallystable. Thus the sleeve target 1 can swing for example laterally, thatis to the sides or horizontally and upwards and downwards, as seen inrelation to the longitudinal direction thereof. It implies that also thetow rod 1 and the tow cable 7 will oscillate laterally. Such anoscillatory movement will in turn result in an oscillatory movement ofthe hit detector 9. The hit detector 9 is in the conventional mannerarranged for indicating in different ways the positions of projectiles,such as the paths or the miss distances thereof, when test firing at thesleeve target 1. Then automatic routines for calculation of theprojectile positions are commonly used, that are for example made byelectronic circuits inside the detector 9. In these routines it ispresupposed or required that the position of the hit detector 9 isknown, and particularly that the position thereof in an angulardirection is known. Commonly it is here assumed, that an angular or rollmovement of the hit indicator 9 does not occur and that a reference inthe hit indicator is always located in some vertical plane, moreparticularly that the indicator body is located exactly underneath thetow rod 1 and the tow cable. Such an assumption is satisfactory in thosecases where the calculations in the indicator are made with a not toohigh accuracy. However, in the circumstance where the requirements onaccuracy are elevated, it may be necessary to acquire information on theposition of the hit detector 9.

For this purpose the tow device and in particular the hit detector areprovided with suitable position determining means, so that in particularthe angular position of the hit indicator can be determined. Aconventional measurement detector for the angular position in relationto for example the horizontal plane could be used. However, such ameasurement detector is generally intended for static or stationarymeasurements and will give an erroneous output signal in thecircumstance that it is subjected to accelerations of the typeexperienced by a hit detector. It can be difficult to compensate forthese errors.

Therefore a measurement arrangement is proposed that is shownschematically in FIG. 2. Here an accelerometer A is attached to thelower part of the hit detector 9, at the largest possible distance fromthe tow rod 5. This accelerometer A is arranged to sense movements inlateral directions or in the tangential direction, that is in directionsperpendicular to a plane passing through the centers of the hitindicator 9 and the tow rod 5, that is in directions which areprincipally perpendicular to the large surfaces of the web device 11.These directions are illustrated by the arrows 13 in FIG. 2. By means ofthe accelerometer A the angular position of the hit indicator 9 can bedetermined if it is presupposed that the oscillatory movement occursabout an attachment point indicated at 15 at the airplane and if thevertical distance to this point is known.

However, the distance to the upper attachment point, about which theoscillation occurs is not known. Also, the oscillatory movement is mostoften composite, so that a plane through the tow cable 7 and the planecentrally through the web device 11 can form an angle that generally issmall. In order to make the measurements independent of the distance tothe upper attachment point another accelerometer B is arranged at thetow rod 1 sensing the acceleration in the same directions as the otheraccelerometer A. These directions are illustrated by the arrows 17 inFIG. 2, the arrows 13 and 17 thus indicating parallel directions.

If it is assumed that the upper attachment point 15 is fixed, theacceleration of the point, where the upper accelerometer B is attached,has the direction as indicated by the arrows 17, that is in a tangentialdirection. The value of the acceleration is thus measured by theaccelerometer B. This acceleration can be written as ##EQU1## where φ isthe angle between a vertical plane passing through the point 15 and aplane passing through the tow cable 5 and in the forward flightdirection and r_(B) is the distance from the upper attachment point 15to the upper accelerometer B, as seen in a horizontal direction.Further, if it is assumed that also the lower accelerometer A is locatedin said plane extending through the tow cable 5 and in the forwardflight direction, that is that there is no composite movement, theacceleration of the point where the lower accelerometer A is attachedalso has a direction as indicated by the arrows 13 (or 17) and theacceleration of this point is thus measured by the accelerometer A. Thevalue of the acceleration of the point where the lower accelerometer Ais attached can then be written ##EQU2## where r_(A) is the distance, asseen in a horizontal direction in said plane, from the upper attachmentpoint 15 to the lower accelerometer A. Then the difference of theaccelerations of the points of the lower and upper accelerometer A, B is##EQU3## where r is the distance in said plane, as seen in a horizontaldirection, between the points where the accelerometers A, B areattached, that is generally the depth or width of the web plate 11, andis thus known and constant. Thus the measured accelerations can besubtracted and then divided by this depth or width r to form a value ofthe second derivative of the angle φ in regard of time. This value canthen be integrated twice for forming a value of the angular position φ.

In the case where the upper attachment point 15 moves in variousdirections and thus has an acceleration, this movement will besuperposed on the case discussed above. However, an equal amount willthen be added to the accelerations of the points where theaccelerometers A, B are attached. By making the subtraction as describedabove, this equal added amount will not influence the result of thesubtraction.

Thus, for determining the angular position the signals from the twoaccelerometers A and B can be provided to a subtraction circuit 19, seethe block diagram of FIG. 3. By means of the subtraction components inthe signals from the accelerometers can be compensated, for example asdiscussed above but also possibly some noise signals, etc. Thedifference signal formed by the subtraction circuit 19 is provided to ananalog to digital converter 21 where the difference signal is convertedto a digital shape and is provided to a microprocessor 23. In themicroprocessor 23, in integration routines 25 therein, the differencesignal is integrated twice for determining the angular position of thehit detector or hit indicator 9, that is for determining the angle, thata plane passing centrally through the hit indicator 9 and the tow rod 5forms to the vertical plane.

In the microprocessor 23 also calculations are made, based on signalsfrom the hit indicator 9, for determining intended hit parameters in acalculation block 27 comprising routines 29 for compensation orconsideration of the position of the hit indicator 9.

I claim:
 1. A method of determining hit parameters of a projectilepassing in the vicinity of a target provided with a hit indicatorcomprising an indicator body suspended in a tow connection, by means ofwhich the target is intended to be towed, the method comprising thesteps of:detecting, at the hit indicator, shock waves generated by theprojectile, determining a value of an angular position of the hitindicator body at instances when the shock waves are detected, making,based on results obtained in the step of detecting and on the value ofthe angular position, calculations for determining the hit parameters,wherein, in the step of determining a value of the angular position, thevalue of the angular position of the hit indicator body is determined byfirst measuring a body acceleration of the hit indicator body in a bodymeasuring direction perpendicular to a plane through the indicator bodyand the tow connection and thereupon integrating the measuredacceleration for finding a value of the angular position.
 2. The methodof claim 1, wherein, in the step of determining a value of the angularposition of the hit indicator body, a connection acceleration at aconnection place located between the indicator body and the towconnection is measured in the body measuring direction, and thereupon adifference of the measured body acceleration and the connectionacceleration is integrated.
 3. The method of claim 1, wherein, in thestep of determining a value of the angular position of the hit indicatorbody, the angular position is determined in relation to a reference thatis fixed to the earth.
 4. A device for determining hit parameters of aprojectile passing in the vicinity of a target, the device comprising:ahit indicator connected to the target for detecting shock wavesgenerated by the projectile, the hit indicator comprising an indicatorbody suspended in a tow connection, by which the target is intended tobe towed, an angular position determining means comprising:a firstaccelerometer attached to the hit indicator body for providing a firstsignal representing the acceleration of the hit indicator body in a bodymeasuring direction perpendicular to a plane through the indicator bodyand the tow connection, an integration means connected to the firstaccelerometer for receiving the first signal and for deriving from thefirst signal a value of an angular position of the hit indicator body,and a calculating means connected to the hit indicator and to theangular position determining means for making, based on the detecting ofthe shock waves and the value of the angular position, calculations todetermine the hit parameters.
 5. The device of claim 4, wherein theangular position determining means further comprises:a secondaccelerometer attached at a connection place between the hit indicatorbody and the tow connection for providing a second signal representingthe acceleration in the body measuring direction at the connectionplace, and a difference forming means connected to the first and secondaccelerometers to receive the first and second signals and to form adifference signal representing the difference between the first andsecond signals, the difference forming means being further connected tothe integration means for providing the difference signal to theintegration means and the integration means using the difference signalin deriving the value of the angular position.
 6. The device of claim 4,wherein the angular position determining means are arranged to determinethe angular position of the hit indicator body in relation to areference fixed to the earth.